In order to accommodate the demands such as miniaturization or advancement in performance of various electronic devices, miniaturization and high integration of printed wiring boards and various electric/electronic elements have progressed. As a result, in terms of such printed wiring boards and electric/electronic elements, metal wirings have been thinned, and the distance between wirings have been narrowed. In relation to such printed wiring boards and various electric/electronic elements, metals such as copper, silver, tin, lead, zinc, aluminum, nickel, gold, or alloys thereof, which are excellent in conductivity and workability, have been widely used as materials for the wiring.
When printed wiring boards and electric/electronic elements are processed into various devices, the wiring made of metal is often heated once or plural times by surface mounting, or a baking treatment in the case where an insulating layer is formed on a circuit using a photosensitive composition or the like. The heating of a wiring made of metal has the following problems.
First, there is a problem that a surface of the wiring is oxidized by the heating of the wiring. Since metal is widely different from metal oxide in resistance values, when a surface of the circuit is oxidized, electric resistance of the circuit is likely to vary. Such variation in electric resistance of the circuit exerts a large influence on the performance of the product. When the surface of the circuit is oxidized, wettability between solder and the surface of the circuit deteriorates, leading to difficulty in soldering on the circuit.
When the wiring made of metal is heated, dendritic crystals made of a metal compound are likely to be formed as a result of elution (migration) of metal ions on a surface of the substrate due to moisture of a surface of the wiring. When the distance between the wirings is small, there is a problem that dendritic crystals made of the metal compound are formed by migration, and thus short circuit of the wiring is likely to occur.
In order to solve the problems mentioned above, there has been proposed a method in which metal is subjected to a surface treatment before heating. Specifically, there have been proposed a method in which a wiring made of copper or an alloy containing copper is subjected to a surface treatment using a surface treatment liquid containing an imidazole compound, iron ions, and a phosphonic acid-based chelating agent (Patent Document 1), and a method in which a wiring made of copper or an alloy containing copper is subjected to a surface treatment using an aqueous solution of an azole compound as a surface treatment liquid (Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2014-101554
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2012-244005